Method of removing copper-containing iron oxide incrustations from ferriferous surfaces



Jan. 22, 1963 G. s. GARDNER 3,074,825

METHOD OF REMOVING COPPER-CONTAINING IRON OXIDE INCRUSTATIONS FROM FERRIFEROUS SURFACES Flled Apnl 3, 1962 2 Sheets-Sheet 1 at U.) E 0.00 .J O 2 I g ,7. FIGJ. MINIMUM AMOUNT MTU REQUIRED '5 TO PREVENT cu Pun-me IL! g CONDITIONS 8 0.000, TEMPERATURE 160 F. lMMERSION 2 News U ACID, Hct. CORROSION INHIBITOR 5% by M.

filyoR METHYLOL THIOUREA CONCENTRATION-MOLS/LITER wamfw A'ITORN EYS Jan. 22, 1963 Filed April 3, 1962 CLOOOI GARDNER 3,074,825

G. S. METHOD OF REMOVING COPPER-CONTAINING IRON OXIDE INCRUSTATIONS FROM FERRIFEROUS SURFACES 2 Sheets-Sheet 2 FIG. 2.

MINIMUM AMOUNT DMTU REQUIRED TO PREVENT CU PLATING CONDITIONS TEMPERATURE I60" F. IMMERSION zuouns ACID 5% by wt CORROSION mmarroR 0.1 by wt INVENTOR mMs'rHvLoL THIOUREA CONCENTRATION -MOLS/UTER ATTORNEYS 3,074,825 METHOD OF REMOVING COPPER-CONTAINING IRON OXIDE INCRUSTATIONS FROM FERRIF- EROUS SURFACES George S. Gardner, Elkins Park, Pa., assignor, by mesne assignments, to Chemical Cleaning, Inc., a corporation of Louisiana Filed Apr. 3, 1962, Ser. No. 184,736 8 Claims. (Cl. 134-41) This invention relates to the art of removing coppercontaining iron oxide incrustations from ferriferous metal surfaces and is particularly useful in connection with the removal of such incrustations from the internal surfaces of industrial heating equipment such, for example, as boilers, radiators, condensers and generators. In this art, as is well known, it is common practice to employ aqueous solutions of hydrochloric or other acids which are capable of dissolving the incrustations, which incrustations include iron oxide and certain salts coming from impurities in the water plus a small but troublesome content of copper picked up from various sources familiar to those skilled in this art.

The removal of such copper-containing iron oxide incrustations, especially from industrial equipment of the type referred to, involves certain difliculties. For instance, use of hydrochloric or other acid solutions per se is not always fully effective because, although the iron oxide or other contaminants are readily dissoived by the acid solution, the copper component of the incrustations is sometimes merely redeposited as a plating on the ferriferous metal surfaces. This problem is referred to in US. Patent 2,567,835 which relates to a method for removing copper-containing boiler incrustations without redeposition of the copper. However, the technique described in this patent involves a number of disadvantages in that it requires a multiplicity of steps which are not only laborious but also excessively time-consuming. For instance, according to the disclosure in the patent referred to, copper-containing incrustations are removed by subjecting them first to the action of a hydrochloric acid solution containing a corrosion inhibitor followed by a water rinse. Secondly, the surfaces are subjected to the action of an ammoniacal solution containing a strong oxidizing agent followed again by a water rinse. As a third step, they are subjected to the action of a dilute hydrochloric acid solution and finally, as a fourth step, to the action of a hot dilute aqueous alkaline solution. While this technique ultimately accomplishes the desired objective, namely the removal of the incrustation including its copper component yet, as noted above, it is complex, laborious, time-consuming and expensive.

More recently, it has been discovered that such coppercontaining iron oxide incrustations can be removed through use of an aqueous acid solution containing certain urea derivatives, for example, thiourea, 1,3-dimethylthiourea, ethylene thiourea, 1,3-diethylthiourea, and 1,3-diisopropylthiourea. Although this process is capable of removing the iron oxide deposits Without redeposition of the copper component, such thioureas are not capable of functioning per se as corrosion inhibitors, and they interfere with the inhibiting action of other corrosion inhibitors which may have been added to the acid solutions, thereby allowing undesirable acid attack on the basis metal under the incrustation.

With the foregoing in mind, the principal object of the present invention is the provision of a process for removing copper-containing iron oxide incrustations from ferriferous surfaces and more especially from the interior surfaces of industrial heating equipment, which process involves the use of acid solutions containing cer- Patented Jan, 22, 1963 tain agents which in no way interfere with the action of any corrosion inhibitors which may have been employed and which agents under certain circumstances may, in themselves, actually provide an excellent measure of corrosion inhibition.

Other objects and advantages of this invention will become apparent in considering the following disclosure and the accompanying drawings wherein FIGURE 1 is a semi-logarithmic graph illustrating the use of monomethylol thiourea in the acids employed in the removal of copper-containing incrustations; and

FIGURE 2 is a semi-logarithmic graph illustrating the use of dimethylol thiourea under conditions similar to those illustrated in FIGURE 1.

Under the conditions of both FIGURES 1 and 2 the quantities of thiourea involved are those which are required for essentially complete removal of the copper content under the conditions of the tests.

The present invention is based upon the discovery that a methylol thiourea having the formula:

wherein X is selected from the group consisting of hydrogen and CH OH, when added to the acid solution employed makes it possible to effect complete removal of copper-containing iron oxide incrustations from metal surfaces without redeposition of the copper ion and with.- out interfering with the action of any organic nitrogenbase corrosion inhibitor which also may have been employed.

Acids which are known to be suitable for use in dissolving copper-containing iron oxide incrustations include mineral acids such as hydrochloric, phosphoric and sulfuric, also sulfamic acid, as well as organic acids such as acetic, oxalic, citric and gycolic. These acid solutions, containing a methylol thiourea either with or without a suitable corrosion inhibitor, may be brought into contact with the iron oxide incrustations in any convenient manner. For example, when treating the inside of a boiler the latter may simply be filled with the solution of the present invention so as to contact the area of incrustation. Where very large vessels are tobe treated it is generally more economical to direct a stream of the acid treating solution against the incrustation instead of filling the vessels.

While my process may be at least partially useful at lesser or greater temperatures, I have found that the desired copper and iron oxide removal action, without redeposition or plating of copper, is obtained to an optimum extent at temperatures between about 75 F. and 200 F. Because of this temperature requirement, it may be necessary in some cases to supply heat to the solution or to the equipment being cleaned, or to both the solution and the equipment in order to insure having the incrustations and the solution in contact with each other at a temperature at which the incrustations will be acted upon by the solution and thereby removed.

So far as concerns the amount of methylol thiourea compound to be employed in the process of this invention, it has been found that this will depend upon several factors. For example, where the monomethylol thiourea compound is employed, generally smaller amounts thereof are required on a molar basis, per mol of copper ion sought to be removed, than would be required when using the dimethylol thiourea compound.

Variations in the acid systems utilized, the acid concent-rations, the temperature, as well as the organic corrosion inhibitor employed, if any, all tend to influence the amount of methylol thiourea compound to be used. Varying amounts of copper ion in the'solution also require different concentrations of methylol thiourea compounds, praticularly where complete copper removal is desired. Nevertheless, generally speaking, there is a semi logarithmic relationship which has been found to exist between the minimum amount of the methylol thiourea compound which should be employed and the aomunt of copper to be removed by the solution provided, of course, that other variables remain substantially constant. The accompanying graphs show the relationship between the thiourea compound employed and the copper to be removed, the graph of FIGURE 1 showing this relationship for methylol thiourea, and the graph of FIGURE 2 for dimethylol thiourea. The other factors, namely the temperature, the period of treatment (immersion), the type of acid and its concentration and the content of inhibitor employed, are all constant as shown by the graphs.

In developing both graphs the inhibitor employed was prepared in accordance with Example I of US. Patent 2,758,970.

At this point I wish to explain what I mean by copper remova The copper, of course, forms a part of the incrustation which must be removed from the surface of the work being treated and this must be done without redeposition of the copper as a plating on the surface. However, as an actual matter of fact, it is my belief that the action is more one of a chelating procedure, i.e., the dissolved copper ion is in some way heldor retained in the solution in a relationship which prevents its redeposition on the surface. In presenting this theory of the action involved, it should be understood that I am not fully aware of just what takes place except that the copper ion is not redepositioned upon the surface when the teachings of this invention are employed. However, this theory is substantiated by the results of the tests which Ixhave conducted, as shown in the tables hereinafter introduced. In making the tests it was, of course, impossible to obtain pieces of ferrous metal carrying incrustations of the kind with which the invention is intended to deal. For this reason the acid solutions employed in the tests were prepared to include certain copper concentrations as set forth and the concentrations utilized are strictly representative of copper concentrations which are generally encountered in the removal of copper-containing iron oxide incrustations of the kind referred to. The minimum amount of methylol thiourea compound required in the solution to prevent copper from depositing upon ferrous metal pieces is plotted in the graphs.

It will be observed for both graphs that, under the conditions employed, the amounts of methylol thiourea compounds required per unit of copper ion present are larger at very low copper concentrations, but that, as the copper concentration increases, the amount of the methylol thiourea which is required is not proportionately increased.

Generally, it has been found that use of approximately 4 mols of methylol thiourea compound per mol of copper provides essentially complete copper removal. No maximum limitation has been found as to the quantity of methylol thiourea which can be used and as much as 16 mols of the compounds of this invention per mol of copper have yielded completely satisfactory results. However, in the interests of economy and to prevent waste it is preferred to employ no more than about mols of methylol thiourea compound per mol of copper contained in the iron oxide incrustations.

' The amount of copper contained within the iron oxide incrustations may, of course, be determined beforehand by analytical methods rfamiliar to the art.

As noted hereinabove, the methylol thiourea derivatives found to be suitable for use in the process of this invention introduce no apparent deleterious effect upon the corrosion inhibitors employed in the acid solutions, but actually in themselves impart an excellent degree of corrosion inhibition with respect to the metal and even provide some synergistic eifect with other corrosion inhibitors which may be used so that the use of corrosion inhibitors per se may be desirable depending upon the particular acid employed. Where such additional inhibitors are used it is, of course, essential that they be soluble in the acidic solutions.

Well known types of organic nitrogen-base corrosion inhibitors which may be incorporated in the solutions of this invention include the aromatic and heterocyclic coal tar bases produced from the distillation of coal tar. Typical examples of these are pyridine, alpha picoline, beta picoline, gamma picoline, 2-n-amylpyridine, 4-n-amylpyridine, 2-hexyl-pyridine, various substituted lutidines, collidines, quinolines, lipidines, and quinaldines. The well known rosin amine based inhibitors are also suitable for use in this respect and, of course, there are, many others familiar to those skilled in the art. The concentration or amount of the corrosion inhibitor depends upon its eifectiveness in the particular acid solution employed and at the temperature at which it is used. Generally, the amount effective to inhibit or reduce the corrosive effect of the acid is in the range of 0.1 to 1% based on the weight ofthe acid in the treatingsolution.

In order to illustrate the excellent copper removing or chelating properties of the mono and dimethylol thiourea compounds of the present invention there is presented below, in Table I, a series of test results wherein hot-'. rolled steel (SAE 1010) test strips were subjected to the action of 5% by weight hydrochloric acid solutions, some of which had dissolved therein 0.001 mol/liter ofcopper ion, 0.01 mol/ liter of a methylol thiourea compound as indicated by the designation MTU or DMTU for the mono or dimethylol thioureas, respectively, and/0r 0.1% inhibitor identified in the table by letters in accordance with the following identification:

A=Inhibitor prepared in acocrdance with Examplel of US. Patent 2,758,970.

B=Inhibitor prepared in accordance with Formula 2 of US. Patent 2,807,585. I

C=Inhibitor prepared in accordance with US. Patent 2,510,063 and containing 15 mols of ethylene oxide.

All of these tests were conducted at 115 F. for a six hour period.

Table I Methylol Mol/l. Corrosion Weight, thiourea Cu ion inhibitor loss, lbs./ Cu deposition compound iii/24 hrs.

0. 001 None Gain-Heavy copper plating None do 0. 0139 None. None dn 0. 0067 D0 None A 0.0026 D0 0. 001 A 0. 0009 D0 None A 0. 0026 D0 0. 001 A. 0 0011 D0 None B- 0 0052 Do 0. 001 B. 0 0006 Do None B. 0 0044 Do 0. 001 B- 0 0006 D0 None 0- 0 0061 Do 0.001 O. 0 0010 Do None G 0 0049 Do 0. 001 C 0 0010 Do As can be seen from the data presented above, the use of either the mono or dimet-hylol thiourea derivatives of this invention successfully prevents copper deposition in uninhibited as well as in inhibited acid solutions. Moreover, these data show a surprising reduction in weight loss of the treated metal when the mono or dimethylol thiourea compounds of this invention are used in' conjunction with an acid corrosion inhibitor for removing copper ion from acid solutions.

Hot-rolled steel test strips, similar to those employed in developing the data reported in Table I, were exposed to 5% hydrochloric acid solutions for periods of 16 hours at -115 F. These acid solutions were prepared so as to contain, where indicated, 0.001 mol/liter of copper ion, 0.01 moi/liter of a methylol thiourea compound of this invention, and 0.1% by weight of a corrosion inhibitor prepared in accordance with Example II of U.S. Patent 2,403,153. The results of these tests are reported The MTU and DMTU are, respectively, monomethylol thiourea and dimethylol thiourea.

The results in Table II are a further demonstration of the ability of the methylolated thiourea compounds of this invention to prevent deposition of copper ion from acid solutions on the metal surfaces under treatment. These results also reflect the reduced weight losses obtained on the treated metal when the compounds of this invention are used in conjunction with corrosion inhibitors.

In order to illustrate the comparable copper removing or chelating properties of the mono and dimethylol thiourea compounds of this invention in comparison with the prior art thiourea and lower alkyl substituted thiourea derivatives, there is presented below in Table III a series of test results wherein various amounts of these thiourea compounds were employed under otherwise identical conditions. Each test solution contained 5% by weight oi hydrochloric acid, 0.01 mol/ liter of copper ion and 0.1% by volume of a rosin amine based corrosion inhibitor such as described in Example I of U.S. Patent 2,758,970. All test solutions were maintained at 160 F. and the tests continued for three hours, and the test metal was SAE 1010 hot-rolled steel, previously cleaned by pickling in hydrochloric acid solution.

Table III Thiourea compound Weight loss, Cu deposition lbs./ft. /24 Type Mel/1 hrs.

0. 0033 Heavy.

03 0.0332 Slight. 0.05 0. 0408 Do. 0.07 0. 0443 Do. 0. 05 0. 0110 None. 0. 05 0. 0361 Slight. 0.03 Gain-Heavy copper plating 0. 05 0. 0025 None. 0. 07 0038 Do. 0. 03 0. 0006 Do. 0.05 0.0009 Do. 0. 07 0. 0011 D 0.

6 at uniform concentrations of 0.01 mol/liter of test solution. Copper ion, at 0.001 mol/liter, was used where indicated, and 0.2% by weight of commercial corrosion inhibitors were used as indicated. These inhibitors are identified in Table IV by the letters A and B. Inhibitor A was prepared in accordance with Example I 01f U.S. Patent 2,758,970 as follows: 217 grams of a commercial grade of dehydrogenated rosin amine (approximately 0.70 rnole) was placed in a round-bottom Pyrex flask. 4 moles (293 ml.) of acetone and 2.2 moles (163 ml.) of 37% formaldehyde solution was then added thereto. The mixture was slowly stirred and 0.70 mole (7-0 ml.) of 20 B. hydrochloric acid was added in small increments over a period of 10 minutes. The mixture was gently refluxed for 15 hours, at the end of which time the liquid was distilled to a vapor temperature of C., 200 ml. of distillate being obtained. The distillate contained excess acetone plus some water and formaldehyde. The residue in the flask consisted principally of a solution of:

OHz-OHzOOCH RN -Hoi CH2CHzCO-CHa which is a dark, oily, viscous material. This material was diluted with water to make a total weight of 620 grams.

Inhibitor B was prepared in accordance with U.S. Patent 2,510,063 as cfollows: 15 moles of ethylene oxide were reacted with dehydrogenated rosin' amine to produce a compound having the general formula:

(CH2oH20)mH where R is the dehydroabietyl group, and where X is the -(CH CH O),,H group, and wherein the sum of m +n is 15.

Table IV Methylol thiourea Cu Corrosion Weight loss Cu deposi compounds present inhibitor lbs./lt. /24 tion present hrs.

Thlourea A 0. 0268 None. Diethylthiourea A 0. 0429 Do. Ethylenethiourea A 0. 0265 Do. Methylol thlourea. A 0. 0135 D o. Dimethylol thiourea A 0. 0093 Do. Thiourea .4. 0. 1042 Slight. Diethylthiourea A 0.0467 None. Ethylenethionrea A 0.0450 Do. Methylol thiourea. A 0. 0019 Do. Dimethylol thiourea A 0.0018 Do. Thiourea B 0. 1742 Do. Diethylthioure B 0. 1575 Do. Ethylenethlourea B 0. 1910 Do. Methylol thiourea- B 0. 1050 Do. Dtmethylol thiourea. B 0. 0705 Do Thiourea B 0. 1442 Slight. Diet-hylthiourea B 0. 0515 None. Ethylenethiourea B 0. 0501 Do. Methylol thiourea. B 0. 0021 Do. Dimethylol thiourea.-. B 0. 0050 Do.

From Table IV it is apparent not only that copper deposition is prevented in acid solutions with the methylol- Iated thiourea compounds of this invention, but also that greatly reduced corrosion loss is achieved through use of these compounds.

Moreover, attention is directed to the increase in weight loss obtained from the incorporation of inhibitor A with the three prior art thiourea compounds. These results show clearly the antagonistic effects of these particular thiourea derivatives with a commercial acid corrosion inhibitor.

The use of other acid systems for dissolving coppercontaining iron oxide incr-ustations from ferriferous surfaces, some of which systems also contained 0.1 mol/ liter of methylol thiourea, is shown below in Table V. All of these solutions were run at F. for 45 minutes and contained 0.01 incl/liter of copper ion.

. The data presented above demonstate that the methylolated thiourea derivatives of this invention exhibit inhibitory properties in themselves and are completely capable of removing copper from iron oxide incrustations found on the internal surfaces of industrial heating equipment by what is believed to be a chelation process whereby the methylolated thiourea forms a complex withthe copper ion, thereby preventing its redeposition from the acid solution onto the ferrous metal sunfaces.

Although the data reported in this disclosure were the results of laboratory tests, I wish it to .be distinctly understood that these results have been fully confirmed as correct by experimental work in the field.

I claim:

\1. In the art of removing copper-containing iron oxide incrustations from ferriferous surfiaces by subjecting the surfaces to the action of an aqueous acid solution capable of dissolving the incrustations, the method which comprises adding to the solution a soluble methylol thiog urea having the formula v e wherein 'X is selectedfrom'the groupconsisting of by drogen and CH OH and then treating the surface with the solution to effect removal of incrustations. f 2. The method of claim '1 wherein the quantity of methylol thiourea employed is not less than about 2 mols for each mol of copper in the incrustation.

3; The method of claim lwhe-rein the temperature of the acid solution is maintained at from F. to 200 F. 4. The method of claim 2 wherein the temperature of the acid solution is maintained at from 75 F. to 200 F. 5. The method of claim 2 wherein the. quantity of methylol thiourea is from 2 to 10 mols per mol of copper. 6. The method of claim'l wherein the quantity of methylol thioureais from 2 to 10 molsper mol of copper and, further, wherein the temperature of the acid solution is maintained at from 75 F. to 200 F. g 7. The method of claim 1 wherein the acid solution also contains an amount of an organic nitrogen-base other than the methylol thiourea specified, which organic nitrogen-base isvcapable of inhibiting the corrosive actionof the acid on ferriferousmetal. '8. The method of claim 2 wherein the acid solution also contains, an amount of an organic nitrogen-base other than the methylol th-iourea specified, which organic nitrogen-base is capable of inhibiting the corrosive action of the acid on ferriferous metal.

References Cited in the file of this patent UNITED STATES PATENTS 2,959,555 Martin et a1. Nov. 8, 1960 

1. IN THE ART OF REMOVING COPPER-CONTAINING IRON OXIDE INCRUSTATIONS FROM FERRIFEROUS SURFACES BY SUBJECTING THE SURFACES TO THE ACTION OF AN AQUEOUS ACID SOLUTION CAPABLE OF DISSOLVING THE INCRUSTATIONS, THE METHOD WHICH COMPRISES ADDING TO THE SOLUTION A SOLUBLE METHYLOL THIOUREA HAVING THE FORMULA. X-NH-C(=S)-NH-CH2-OH WHEREIN X IS SELECTED FROM THE GROUP CONSISTING OF HYDROGEN AND CH2OH AND THEN TREATING THE SURFACE WITH THE SOLUTION TO EFFECT REMOVAL OF INCRUSTATIONS. 